The present invention generally relates to thermal inkjet printing technology, and more particularly to a multi-color thermal inkjet printing system in which color bleed problems between adjacent printed regions are controlled.
Substantial developments have been made in the field of electronic printing technology. Specifically, a wide variety of highly efficient printing systems currently exist which are capable of dispensing ink in a rapid and accurate manner. Thermal inkjet systems are especially important in this regard. Thermal inkjet systems basically involve a cartridge which includes at least one ink reservoir/compartment in fluid communication with a substrate having a plurality of resistors thereon. Selective activation of the resistors causes thermal excitation of the ink and expulsion thereof from the ink cartridge. Representative thermal inkjet systems are discussed in U.S. Pat. No. 4,500,895 to Buck et al.; U.S. Pat. No. 4,794,409 to Cowger et al.; and the Hewlett-Packard Journal, Vol. 39, No. 4 (August 1988), all of which are incorporated herein by reference.
Recently, additional developments have been made in the field of thermal inkjet technology involving the generation of multi-colored images. This is typically accomplished through the use of specially-designed thermal inkjet cartridges having a plurality of individual ink compartments therein. Each of the compartments is designed to retain a selected ink having specific physical/color characteristics. By combining these ink materials on a substrate (e.g. paper) in varying configurations and quantities, multi-colored images having a high degree of print resolution and clarity may be produced. Exemplary thermal inkjet cartridges having multiple ink-containing compartments are illustrated and described in U.S. Pat. No. 4,771,295 to Baker et. al. and U.S. Pat. No. 5,025,271 to Baker et. al. which are both incorporated herein by reference.
However, under certain circumstances, a significant problem can occur when multi-color images are printed using thermal inkjet technology as described above. Specifically, this problem involves a situation known as "color bleed". In general and for the purposes set forth herein, color bleed is a term used to describe the diffusion/mixture of at least two different colored ink regions into each other. Such diffusion/mixture normally occurs when the different colored regions are printed next to and in contact with each other (e.g. at their marginal edges). For example, if a region consisting of a first coloring agent (e.g. black) is printed directly adjacent to and against another region consisting of a second coloring agent (e.g. yellow), the first coloring agent will often diffuse or "bleed" into the second coloring agent, with the second coloring agent possibly bleeding into the first coloring agent. Accordingly, indistinct images with a poor degree of resolution are produced. An insufficient degree of resolution results from the production of jagged, non-linear lines of demarcation between adjacent colored regions instead of sharp borders therebetween. This can create significant problems, especially when high volume printing systems are used to print complex, multi-color images.
In addition, color bleed problems in multi-ink systems are also caused by strong capillary forces generated in many commonly-used paper substrates. These capillary forces cause a "wicking" effect in which coloring agents are drawn into each other by capillary action through the fibers of the paper materials. This situation also results in a final printed image of poor quality and definition.
The present invention represents a unique and highly effective approach in the control of color bleed in multi-color thermal inkjet printing systems. The methods described herein may be implemented at a minimal cost, and do not require the use of extra equipment, custom-manufactured paper, and/or special paper coatings. The present invention therefore represents an advance in the art of thermal inkjet printing technology as described in greater detail below.